Critical Injector Pressures

Diesel really is oily. It certainly doesn’t burn like Hollywood would portray it when a truck loaded
with Diesel Fuel explodes like an atomic bomb! Well the Diesel Liquid Fuel wouldn’t blow up like that
BUT if it was broken down into a vapour it would. Remember Petrol appears highly explosive but it
really is the vapour that catches on fire well ahead of the liquid. So how then does Diesel Fuel
contain so much energy? In fact it has so much energy stored in it that a modern day small 3 Litre
Diesel engine can have much more Torque than a 6.5 Litre V8 Petrol.

Well it’s possible all due to Injector pressure and design. From a design and spray pattern point of
view the Diesel injector has the job of spraying the fuel proportionally around the combustion
chamber for an even burn. That’s an easy one. Pressure is where it all starts to become a little more
complex. Many people get mixed up when it comes to Injector opening pressure and Injector line
pressure. Most petrol engine injectors open at around 50 PSI whilst a modern day Diesel injector
opens at pressures above 2,000 PSI on average. Common Rail Diesel “Fuel line” pressures climb
rapidly to above 20,000 PSI BUT remember this is the “Fuel line supply pressure” NOT the injector
opening pressure…Common Rail injectors still have springs inside and effectively open at the “usual”
injector pressures. It is just the high supply pressure of Common Rail that blasts the fuel through a
Common Rail injector for even more atomisation again. At any of these high pressures the Diesel
Fuel is broken down to very tiny droplets which brings the Fuel particle closer to that volatile vapour
which is great for burning. Now you can see how Diesel gets its power.

These pressures are set with the injector pump and injector matching in design. This is why
‘modifying’ or ‘raising’ injector pressure above specifications on a older Diesel with a Fuel injection
pump is not always a good thing. To ‘de-bunk’ this theory of ‘wind the Injector opening pressures up
and she’ll get more power’, we did some testing on our old Nissan GQ Patrol with the trusty 4.2
Diesel.

First run was with Injector pressures at a standard 1,600 PSI: No smoke and power was normal.

Second Run was with Injector pressures lowered to 800 PSI: Very smokey and possibly 1 or 2 HP
more power on average. Reason: The lower pressures meant that the injector was not spraying well,
opening earlier and closing later and letting in more fuel generally. Too much fuel meant smoke and
maybe a tiny increase in power offset by all that excess fuel.

Third Run with Injector pressures were raised to 2,400 PSI: No smoke and a lot less power. Reason:
The higher injector pressure was spraying well but opened later and closed earlier lowering the
overall fuel volume. Less Fuel means less power.

In the end it was clear that injector pressures must be kept to within a manufacturer’s specification
to get the best overall power and efficiency.

Fuel supply pressures and the effects of changing them we will chat about next time!